In the technical field described above, multi-antenna systems and multi-antenna transmission methods have been proposed for future mobile communications systems from viewpoints of improvements in speed, quality, etc., of information transmission. According to the multi-antenna systems, data transmission is carried out using two or more antennas for transmission and/or reception, wherein not only frequency and time but also space is effectively utilized. The multi-antenna transmission methods generally include a MIMO (Multiple Input Multiple Output) multiplexing method, a MIMO diversity method, and an adaptive array antenna (AAA: Adaptive Array Antenna) method.
The concept of the MIMO multiplexing method is described with reference to FIG. 1A. The MIMO multiplexing method is a technique for increasing the amount of data transmission by a factor equal to the number of antennas that carry out simultaneous parallel transmission of different data from two or more transmission antennas. For simplicity, two antennas are used by each of a transmission side and a reception side. According to an example that is illustrated in FIG. 1A, separate symbol sequences (symbol sequence A, B, . . . , and symbol sequence C, D . . . ) are transmitted from each antenna. Since the number of the transmission and receiving antennas is two, compared with the case where there is one, twice as great magnitude of data can be transmitted at once. A suitable signal separating method is performed on the reception side. As the signal separating method, a known method in the technical field concerned may be used, for example, a blasting (BLAST) method, a MMSE method, and a MLD method. In order to obtain a good quality data transmission by the MIMO multiplexing method, a relationship of NTX<=NRX must be sufficed between the number NTX of the transmission antennas and the number NRX of the receiving antennas. The MIMO multiplexing method is disclosed by, for example, Non Patent Reference 1.
The concept of the MIMO diversity method is described with reference to FIG. 1B. The MIMO diversity method is a technique for improving reliability on the reception side by transmitting two or more streams of the same contents in parallel from two or more transmission antennas. According to an example that is illustrated in FIG. 1B, when transmitting symbols A and B, one antenna transmits in a sequence of B and A, and another antenna transmits in a sequence of A* and −B*. Here, “−” is a negative sign, and “*” represents a complex conjugate. A technique of converting the symbols A and B to be transmitted into two streams such as above is called space-time block coding (STBC), or more simply, a linear process. On the reception side, B+A* and A−B* are received in this sequence, and the symbols A and B transmitted are deducted from the received signals. Since the two signals transmitted from the two antennas are subjected to separate fading, reliability of the received signals is improved by appropriately combining the two signals on the reception side. The MIMO diversity method is disclosed by Non Patent Reference 2, for example.
The concept of the adaptive array antenna method is described with reference to FIG. 1C. According to the adaptive array antenna method, two or more antennas are used, and the same number of copies of the same symbol sequence is produced for transmission, wherein each copy is multiplied by a corresponding transmission weight. By appropriately adjusting the transmission weights, a directional beam that has a main lobe directed to a communication partner is formed, and the communication quality on the receiving side can be raised.
Further, there are techniques of coping with a data error, one of which techniques is a hybrid ARQ (Automatic Repeat Request). This is a combination of a resending request for a packet when an error is detected with an error detection code (CRC: Cyclic Redundancy Check), and an error correction code by error correction coding (channel coding).
FIG. 2 shows a general process of the hybrid ARQ. On the transmission side, a CRC bit is attached (step S1), and error correction coding is performed (step S2). On the receiving side, error correction decoding is performed (step S3), and error detection using the CRC bit is performed (step S4). If an error is detected, a resending request is provided to the transmission side. If no error is detected, the transmission and the reception are completed.
Further, FIG. 3A, FIG. 3B, and FIG. 3C show types of the Hybrid ARQ process. According to the type shown in FIG. 3A, when a decoding error is present in a packet P1, the packet P1 is discarded, and a packet P2 containing the same contents is received for decoding again. According to the types shown in FIG. 3B and FIG. 3C, when a decoding error is present in the packet P1 the packet P1 is not discarded but held. Then, the packet P2 that is resent is combined with the packet P1 such that a packet P3 is generated, and the packet P3 is decoded. According to the type shown in FIG. 3B, the contents of the packet P2 are the same as the packet P1. By combining the packets, reception SIR is improved. According to the type shown in FIG. 3C, the resent packet is punctured with a different pattern. In this way, coding gain is improved by combining the packets.
Further, types of the MIMO multiplexing method are described with reference to FIG. 4A and FIG. 4B. According to the type shown in FIG. 4A, transmission antennas #1 and #2 transmit transmitting data (transmitting streams) #1 and #2, respectively. According to the type shown in FIG. 4B, the transmitting data (transmitting streams) #1 and #2 are weighted by w1,1, w1,2, w2,1, and w2,2, and provided to the transmission antennas #1 and #2, respectively, such that antenna beam patterns are formed corresponding to the transmitting data (transmitting stream) #1 and #2. The present invention is applicable to both types.    [Non Patent Reference 1] G. J. Foschini, Jr.: “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas”, Bell Labs Tech. J., pp. 41-59, Autumn 1996    [Non Patent Reference 2] V. Tarokh, H. Jafarkhani, and R. Calderbank: “Space-Time Block Coding for Wireless Communications: Performance Results”, IEEE J. Select. Areas Commun., Vol. 17, No. 3, pp. 451-460, March 1999    [Non Patent Reference 3] Paulraj et al., “Transmit Optimization for Spatial Multiplexing in Presence of Spatial Fading Correlation”